The present invention generally relates to an atrial defibrillator and method for applying cardioverting electrical energy to the atria of a human heart in need of cardioversion. The present invention is more particularly directed to a fully automatic implantable atrial defibrillator which exhibits improved safety by reducing the potential risk of induced ventricular fibrillation which may result from the mistimed delivery of cardioverting electrical energy to the atria. More specifically, the atrial defibrillator and method of the present invention guards against applying cardioverting electrical energy to the atria under conditions believed to contribute to induced ventricle fibrillation.
Atrial fibrillation is probably the most common cardiac arrhythmia. Although it is not usually a life threatening arrhythmia, it is associated with strokes thought to be caused by blood clots forming in areas of stagnant blood flow as a result of prolonged atrial fibrillation. In addition, patients afflicted with atrial fibrillation generally experience palpitations of the heart and may even experience dizziness or even loss of consciousness.
Atrial fibrillation occurs suddenly and many times can only be corrected by a discharge of electrical energy to the heart through the skin of the patient by way of an external defibrillator of the type well known in the art. This treatment is commonly referred to as synchronized cardioversion and, as its name implies, involves applying electrical defibrillating energy to the heart in synchronism with a detected ventricular electrical activation (R wave) of the heart. The treatment is very painful and, unfortunately, most often only results in temporary relief for patients, lasting but a few weeks.
Drugs are available for reducing the incidence of atrial fibrillation. However, these drugs have many side effects and many patients are resistent to them which greatly reduces their therapeutic effect.
Implantable atrial defibrillators have been proposed to provide patients suffering from occurrences of atrial fibrillation with relief. Unfortunately, to the detriment of such patients, none of these atrial defibrillators have become a commercial reality.
Implantable atrial defibrillators proposed in the past have exhibited a number of disadvantages which probably has precluded these defibrillators from becoming a commercial reality. Two such proposed defibrillators, although represented as being implantable, were not fully automatic, requiring human interaction for cardioverting or defibrillating the heart. Both of these proposed defibrillators require the patient to recognize the symptoms of atrial fibrillation with one defibrillator requiring a visit to a physician to activate the defibrillator and the other defibrillator requiring the patient to activate the defibrillator from external to the patient's skin with a magnet.
Improved atrial defibrillators and lead systems which exhibit both automatic operation and improved safety are fully described in copending U.S. applications, Ser. No. 07/685,130, filed Apr. 12, 1991, in the names of John M. Adams and Clifton A. Alferness for IMPROVED ATRIAL DEFIBRILLATOR AND METHOD and Ser. No. 07/856,514, filed Mar. 24, 1992, in the names of John M. Adams, Clifton A. Alferness, and Paul E. Kreyenhagen for IMPROVED ATRIAL DEFIBRILLATOR, LEAD SYSTEMS, AND METHOD, which applications are assigned to the assignee of the present invention and incorporated herein by reference. As disclosed in the aforementioned referenced applications, synchronizing the delivery of the defibrillating or cardioverting electrical energy to the atria with a ventricular electrical activation (R wave) of the heart has been considered important to avoid cardioverting the heart during the heart's vulnerable period or T wave to thus prevent induced ventricular fibrillation. Ventricular fibrillation is a fatal arrhythmia which can be caused by electrical energy being delivered to the heart at the wrong time in the cardiac cycle, such as during the T wave of the cycle. The atrial defibrillators of the aforementioned referenced applications exhibit improved safety from inducing ventricular fibrillation by sensing ventricular activations of the heart in a manner which avoids detecting noise as ventricular electrical activations for generating reliable synchronization signals. Hence, these implantable atrial defibrillators, by providing such noise immunity in R wave detection assure reliable synchronization.
Another measure for reducing the risk of inducing ventricular fibrillation during the delivery of cardioverting electrical energy to the atria of the heart employed by the defibrillators of the aforementioned referenced applications is the reduction of the amount of the electrical energy which is passed through the ventricles during cardioversion of the atria. This is achieved by locating the cardioverting electrodes in or near the heart to provide a cardioverting energy path which confines substantially all of the cardioverting electrical energy to the atria of the heart.
It has also been observed that during episodes of atrial fibrillation, the cardiac rate increases to a high rate and/or becomes extremely variable. At high cardiac rates, the R wave of each cardiac cycle becomes closely spaced from the T wave of the immediately preceding cardiac cycle. This creates a condition known in the art as an "R on T" condition which is believed to contribute to induced ventricular fibrillation if the atria are cardioverted in synchronism with the R wave close to the preceding T wave. For highly variable cardiac rates, a long cardiac cycle can be followed by a relatively short cardiac cycle. This condition is believed to cause dispersion of refractoriness and also can result in a vulnerable R on T condition. For a more complete understanding of the aforementioned highly variable cardiac rate and the consequences thereof, reference may be had to an article entitled El-Sherif et al., Reentrant Ventricular Arrhythmias in the Late Myocardial Infarction Period: Mechanisms by Which a Short-Long-Short Cardiac Sequence Facilitates the Induction of Reentry, Circulation, 83(1):268-278 (1991).
It has been further observed that as ventricular activations propagate through the heart, the right ventricle apex activates slightly prior to the superior left ventricle. Hence, if the atria are cardioverted in synchronism with an activation of the right ventricular apex, the activation of the superior left ventricle may not be completed. Recent studies have shown that cardioverting the atria when the ventricular activation is not fully completed may contribute to induced ventricular fibrillation.
The atrial defibrillator and method of the present invention greatly reduces the risk of inducing ventricular fibrillation during atrial cardioversion or defibrillation by assuring that the cardioverting electrical energy is not applied to the atria during both an R on T condition and a not yet completed ventricular activation. As will be seen hereinafter, this is accomplished by delaying the application of the cardioverting or defibrillating electrical energy until a predetermined delay time has elapsed after detecting a ventricular activation. The delay time is chosen to be of sufficient duration to assure that the ventricular activation has fully completed and of insufficient duration to avoid applying the cardioverting electrical energy to the atria during the T wave immediately following the detected ventricular activation. As a result, a reduced risk of induced ventricular fibrillation is assured because the cardioverting electrical energy will not be applied to the atria during an R on T condition, during an uncompleted ventricle activation, or during a vulnerable period of the heart.